Galliford Try is currently part way through a contract to build a road for a new development on the site of Rugby’s radio mast site, where unexpected groundwater has not held the scheme back.
Work on a new road that will unlock residential potential of a site formerly occupied by radio masts is now nearing completion but, despite its short 2.5km length, has called for some major ground engineering work. Galliford Try is undertaking the £18M contract for the wider scheme that will deliver the project which includes the road, two junctions and a new bridge crossing a canal.
The new road will connect Rugby town centre with a new development called Houlton - a 486ha urban extension which, when complete, will have 6,200 new homes, three primary schools, a secondary school, a GP health centre as well as 200ha of open space.
Once home to 12 radio transmission masts which helped pioneer global communications in the 1920s, the site is mainly greenfield to the south east and to the north west, closer to Rugby, it was allotments and waste ground.
The route of the new road, which includes the two junctions, runs alongside the Oxford Canal and at the north west end of the site, a new bridge crossing the canal is under construction in an area called “Butlers Leap”.
“The varying ground conditions across the project were softer than expected,” explains Galliford Try project manager Paul Naughton. “There is a lot of sand in this area and it was softer than we originally expected and wetter too.
“At Butlers Leap it was pretty complex,” explains Naughton of the new bridge. “The ground conditions across the site have been challenging, mainly consisting of sands and gravels, and we found that the original ground investigation didn’t tell the entire story, especially around the bridge site.”
FK Lowry carried out the piling for the beams to support the canal bridge and installed 29 900mm diameter bored piles to a depth of 27m to support the abutment wall.
“The original scope was for piled foundations, but we went back and undertook additional testing and revised the designs for additional depth of pile as well,” adds Naughton.
The original boreholes undertaken went down to a depth of 23m, however the piles went down to a depth of 27m, with +3m for the working platform.
“The poor ground conditions meant that the piles were fully cased and concrete was poured using a tremie,” says Naughton. “The piles were cropped ahead of abutment construction which took place over the summer.
Before the new bridge could be installed, the team had to prepare for addition of about 15,000m³ of fill to the area surrounding the canal bridge which included a new 5m-high retaining wall and embankment and the extension of an existing culvert.
“Once the bridge beams are in place Rock & Alluvium started installing CFA bearing piles for the bridge wingwalls and retaining wall,” says Naughton. “And we will move the fill around to form the embankment in the spring.”
“We undertook addition pile tests, deeper than the client had originally done and decided to undertake controlled modules columns (CMCs) for the ground improvement instead of the original band drains planned.”
This resulted in a significant time saving for the scheme.
“Rather than wait for fill settlement which would’ve added 16 weeks to the programme, we decided to stabilise the ground by installing about a thousand CMCs, each 300mm in diameter and placed at 1.5. to 2m centres,” explains Naughton.
In addition, a diversion culvert for the Clifton Brook was installed using a 500t crane to lift in pre-cast concrete culvert units to divert the watercourse under the new road.
“We are working in the Clifton Brook to divert and extend the culvert by over pumping the existing brook at the moment,” explains Naughton. “Each of the 88 units weighed 28t.”
It was then that a 1,000t crane lifted in 23 55t beams and two 103t beams into place.
Temporary works for the scheme has included the installation of sheet piled cofferdams by a silent piling rig at five points along the canal to allow for construction of drainage outlets for surface water from the new road which runs roughly parallel.
Galliford has also constructed a retention pond between the road and canal so that volumes of water entering the canal during storms can be controlled.
Liaising closely with the Canals and Rivers Trust, Galliford closed the canal for six weeks to install canal protection sheet piles to enable construction of capping beams and install outfall headwalls. The included a diversion of a rising main.
“It’s a particularly small site close to a main road, canal and houses and we have worked hard to make sure that local people are happy, as well as the canal users. The coordination for all the activities together in about a 200m area has been significant.”
The work is coming to an end at the Butler’s end of the site and when GE visited, the bridge beams were in place and the deck was about to be put on, ready for a spring finish.
For the scheme, Galliford Try has also diverted overhead 11kVs power cables under the route of the road ahead of the site filling during two weekend road closures. The contractor has also created a flood compensation area by shifting 55,000m³ of material to provide a 1 in a 100-year level of flood protection.
“A lot of the material was saturated and we couldn’t use it, so we have had to leave it, stockpile it, and let it dry out,” says Naughton.
“This has changed our methodology and the sequencing of working so we ended up reusing material on later stages in the process that we would have wanted to at the time. It was logistically demanding, but not technically difficult.”
When the scheme is complete around 200,000m³ of material will be used for cut and fill across the site.
At the other end of the new road, where it will connect to the new development, Galliford Try has been constructing embankments and flood alleviation areas in a greenfield area.
“The ground at this end is good blue Lias clay and we have used this for embankment fills and the flood alleviation, again working alongside the canal. Around 80,000m³ of material was removed from the flood alleviation.
“Some of the trial holes did show some water, but as they were conducted in winter so it was considered to be normal. But outside the site boundary we did identify a natural pond which explained the water was coming through the sands.
“This was more than we anticipated from the original survey and should have been simpler, but once we started, the groundwater found its way through every angle. Below the sands we did find a silt layer, and then it went back into sands.”
Galliford’s work will be completed by September this year, and the house builder will continue with its part of the new development. “There has been a lot going on with this project - the bridge, culvert, the logistics, working between green fields at one end of the site and within 10m of houses at the other end, as well as the two junctions in the middle.
“It has been an interesting spread of a scheme. It’s a good variety,” says Naughton. “A bit of everything, proper civil engineering.”